The present invention relates to a system for transmitting motion between the shaft of an internal combustion engine of a motor vehicle and a group of auxiliary devices provided with corresponding drive shafts and comprising an electrical machine capable of acting selectively as a generator and as a motor.
More specifically, the invention relates to a transmission system of the type defined in the introductory part of claim 1 which follows the present description.
There is a known motion transmission system of this type having an electromagnetically operated clutch which is normally energized and engaged to couple the aforesaid pulley to the shaft of the internal combustion engine with respect to rotation. This known system can be used to provide the following functions or operating conditions:                driving the auxiliary devices (air-conditioning system compressor, electrical machine acting as generator, etc.) while the internal combustion engine is running; the electromagnetically operated clutch is engaged and energized;        driving the auxiliary devices, particularly the air-conditioning system compressor, while the internal combustion engine is stopped; in this case, the electromagnetically operated clutch is de-energized and therefore disengaged, and the aforesaid electrical machine is made to operate as a motor to rotate the compressor by means of the aforesaid transmission belt; and        restarting the internal combustion engine (while hot) by means of the aforesaid electrical machine, operating as a motor, for example in order to provide what is known as a stop-start function; in this case, the electromagnetically operated clutch is energized and engaged to couple the said electrical machine to the shaft of the internal combustion engine in the restarting phase.        
The known system described above has the disadvantage of requiring the electromagnetically operated clutch to be permanently energized while the internal combustion engine is running. The prolonged energizing of this clutch creates a consumption of electrical energy which has a negative effect on the overall energy balance.